1. Field of the Invention
The present invention relates to a direction estimating apparatus and method that estimate a direction of an arrival radio signal using an array antenna, and to a directivity controlling antenna apparatus that controls directivity of the array antenna variably based on the estimated result.
2. Description of the Related Art
As techniques that improve communication qualities and frequency channel capacity, attention is recently drawn to antenna directivity controlling techniques that change an antenna directivity dynamically corresponding to propagation environments. A representative example of the antenna directivity controlling techniques is an adaptive array employing an array antenna and digital signal processing.
The adaptive array generally analyzes digital signals received at the array antenna based on some known information, and thereby obtains weights for array elements to form a radiation pattern. One of the known information is a direction of an arrival radio signal. When a direction of a desired signal and that of an interfering signal are known, control is performed to point a beam of the radiation pattern of the array antenna to the direction of the desired signal, and to point a null to the a direction of the interfering signal, whereby it is possible to improve communication qualities.
Meanwhile the technique for estimating a direction of an arrival radio signal is also attractive to detect a position of a communication terminal in a base station. This technique can be used in assigning dynamic channels using spatial traffic information, and further is considered to be applied to an apparatus for supervising unlicensed signals.
An example of methods for estimating a direction of an arrival radio signal with high accuracy from received signals at an array antenna is a subspace-based method represented by a MUSIC (MUltiple SIgnal Classification). The subspace-based method employs eigen vectors of a covariance matrix obtained from complex digital signals received at an array antenna. The details of the MUSIC method is described in "Multiple Emitter Location and Signal Parameter Estimation", R. O. Schmidt, IEEE Trans. AP-34, 3, 1986.
Further proposed method is, for example, a TQR-SVD (Transposed QR-Singular Value Decomposition) that updates eigen vectors sequentially to adapt to a rapid change in propagation environment in a mobile communication. The details of the TQR-SVD method is described in E. M.Dowling, L. P. Ammann, R. D. DeGoat, "A TQR-Iteration Based Adaptive SVD for Real Time Angle and Frequency Tracking", IEEE Trans, SP-42, 4, 1994.
In the case where a direction of an arrival radio signal is estimated using an array antenna, estimation accuracy and resolution at the time a plurality of signals arrival is dependent on, for example, the number of array elements composing the array antenna, a radiation pattern of each array element, and spatial positional relationship between arranged array elements.
It is general to arrange array elements in the form of a circle to estimate a direction of an arrival signal in a horizontal plane. When array elements are arranged on a plane, it is possible to estimate directions of the arrival signal not only in the horizontal plane but also in a vertical plane. In particular, it is possible to improve estimation accuracy in the vertical plane by arranging circular arrays in the vertical direction so as to pile up a plurality of steps, thereby making a cylindrical form as an entire structure.
However piling up the circular arrays in the vertical direction to be the plurality of steps increases the number of array elements, thereby provides a disadvantage in production cost, and further increases a computation amount for signal processing because inputs are increased.